Abstract
Previous studies on European robins, Erithacus rubecula, and Australian silvereyes, Zosterops lateralis, had suggested that magnetic compass information is being processed only in the right eye and left brain hemisphere of migratory birds. However, recently it was demonstrated that both garden warblers, Sylvia borin, and European robins have a magnetic compass in both eyes. These results raise the question if the strong lateralization effect observed in earlier experiments might have arisen from artifacts or from differences in experimental conditions rather than reflecting a true all-or-none lateralization of the magnetic compass in European robins. Here we show that (1) European robins having only their left eye open can orient in their seasonally appropriate direction both during autumn and spring, i.e. there are no strong lateralization differences between the outward journey and the way home, that (2) their directional choices are based on the standard inclination compass as they are turned 180° when the inclination is reversed, and that (3) the capability to use the magnetic compass does not depend on monocular learning or intraocular transfer as it is already present in the first tests of the birds with only one eye open.
Highlights
Each year, migratory birds travel long distances between their breeding grounds and their wintering quarters, and it is well established that they use a light-dependent magnetic compass for orientation [1,2,3,4,5,6,7,8,9,10,11]
In Hein et al [41], we showed that European robins equipped with monocular eye covers that enabled them to see with their left eye only were significantly oriented into their appropriate migratory direction under the north-east in the unchanged magnetic field (NMF) condition (217u627u, r = 0.57, p,0.001, N = 27; Fig. 2A), as well as under the CMF condition (47u645u, r = 0.38, p,0.05, N = 26; Fig. 2B)
Due to the limited amount of experiments that can be performed during one migratory season, we restricted our tests on the left eye open condition, because this was the eye cover condition under which Wiltschko et al [25] had found no magnetic compass orientation
Summary
Migratory birds travel long distances between their breeding grounds and their wintering quarters, and it is well established that they use a light-dependent magnetic compass for orientation [1,2,3,4,5,6,7,8,9,10,11]. Wiltschko et al [26] reported a similar all-or-none lateralization of magnetic compass orientation in favor of the right eye and left brain hemisphere in a diurnally migrating songbird, the Australian Silvereye, Zosterops lateralis. These findings have led to the notion that the vision-mediated magnetic compass is located only in the right eye of migratory birds, whereas input from the left eye only is not sufficient for magnetic compass orientation [25,26,27,28]. The possibility that birds may show uni-hemispheric sleep during flight [32] would favor bilateral perception of magnetic compass directions in a nightmigratory bird
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